1. Field of the Invention
The present invention relates to an image forming apparatus configured to record a toner image on a recording medium according to image data and thermally fix the transferred toner image.
2. Description of the Related Art
In the case of a conventional image forming apparatus that thermally fixes a toner image on a recording sheet, when post-processing is performed on a recording medium, toners on recording sheets stacked on a stack tray may be mutually fused because the temperature of the recording sheets may rise due to heat fixing processing performed during the post-processing. If the toners on the recording sheets are fused, then the recording sheets stacked may adhere to each other. In this case, toner images on the recording sheets may be flaked, and as a result, poor stacking and poor sheet alignment of the recording sheets in the post-processing may occur.
Japanese Patent Application Laid-Open No. 2006-349755 discusses a method for cooling down a recording sheet before post-processing, by cooling a conveyance guiding member using a cooling fan disposed in the vicinity of a sheet discharge port. However, since the method requires a cooling mechanism between a fixing unit and a sheet discharge unit, the method cannot readily be employed in a small-sized machine, which is required to downsize the entire apparatus body and reduce costs.
Japanese Patent Application Laid-Open No. 2003-248349 (corresponding to U.S. Pat. No. 6,788,905) discusses a method for cooling a recording sheet by temporarily delaying timing for discharging onto a stack tray a recording sheet on which a toner is easily fused such as an overhead projector (OHP) film. However, according to this method, in the case of an image forming apparatus in which the toner can be fused on a plain paper, it is necessary to extend a sheet discharge time interval when post-processing is performed on the plain paper. Accordingly, it is very likely that the above-described method cannot satisfy user's desire for a high productivity.
Japanese Patent Application Laid-Open No. 2006-243498 discusses a method for detecting a density of a toner formed on a recording sheet and changing a sheet discharge interval only when the density is at a level that fusing of the toner occurs. However, the method discussed in Japanese Patent Application Laid-Open No. 2006-243498 only focuses on detecting the density of the toner on a recording sheet discharged from an image forming apparatus and cannot determine at a high accuracy whether fusing of the toner occurs.
That is, whether fusing of the toner occurs or not may depend on the density of the toner on a top surface of a recording sheet stack on a stack tray, as well as the density of the toner on the recording sheet discharged from the image forming apparatus. That is, if a large amount of toner is applied on the top surface of the recording sheet stack on the stack tray, then toner fusing easily occurs. In this case, toner images on the recording sheets may be flaked and poor stacking and poor sheet alignment may occur.